1. Field of the Invention
The present invention relates to a carbonaceous acoustic diaphragm and a method for manufacturing the same.
2. Description of the Related Art
The diaphragm of a speaker used in various kinds of audio or video equipment or mobile equipment such as mobile telephones is required to have faithfully reproduce clear sound over a wide range of frequencies, especially, in the high frequency range. Accordingly, the material for the diaphragm must be chosen to satisfy two apparently conflicting properties: high elasticity for providing sufficient stiffness to the diaphragm and low density for reducing the weight of the diaphragm. In particular, in the case of a diaphragm used in digital speakers which have come to attract attention in recent years, the above properties are necessary because of the need for improved vibration response.
In patent documents 1 and 2 cited below, a diaphragm formed from a material produced by uniformly dispersing carbon nanofibers (vapor-grown carbon fibers) and graphite through amorphous carbon is disclosed. However, since the density of this material is as high as 1.0 mg/cm3 or more, in order to achieve the desired acoustic characteristics there is a need to enhance the elastic modulus by increasing the amount of the costly carbon nanofibers and graphite used, and there is also a need to reduce the thickness. This gives rise to the problem that the diaphragm may break during handling, etc., and a problem also arises in terms of productivity.
Patent document 3 discloses a method in which resin powder, which is baked (carbonized) to form glass-like carbon (amorphous carbon), is first heated and spot-fused to form a porous structure which is then carbonized to produce a low-density porous amorphous carbon structure. However, with this method, it is difficult to obtain a porous structure having a high porosity of 40% or higher, and it is not possible to obtain a diaphragm having an overall density of 1.0 g/cm3 or less.
Patent document 4 discloses a carbonaceous acoustic diaphragm fabricated by vapor phase deposition of pyrolytic carbon on a resin-impregnated and carbonized nonwoven or woven carbon fiber fabric. With this method also, it is difficult to obtain a porous structure having a high porosity of 40% or higher.
Patent document 5 discloses an acoustic diaphragm fabricated by etching the surface of a foamed graphite film and impregnating it with plastic. The foamed graphite here refers to the state produced by disrupting the graphite's unique layered structure by gases formed when carbonizing the polymer at high temperatures, and it is difficult to design and control the porosity as desired. Therefore, by impregnating the resin into the foamed graphite and thereby reinforcing the partially thinned defective portions of the graphite, it is attempted to achieve a flat reproduction frequency response; that is, the main purpose is to reinforce the defective portions of the graphite by the resin. Furthermore, since the resin is impregnated by etching the surface, the process is complex, and the process management also tends to become complex.